WO2011119573A1 - Surface treated bioerodible metal endoprostheses - Google Patents

Abstract

An endoprosthesis includes an expandable tubular body defined by a plurality of struts. In some embodiments, the expandable tubular body includes a bioerodible metal that has at least a first surface region and a second surface region. The first and second surface regions can have different surface oxide compositions. In some embodiments, the first portion has a thermally altered microstructure and the second portion has a wrought microstructure. The thermally altered microstructure can be a cast microstructure comprising dendritic grains. The first portion forms at least a portion of an outer surface of the expandable tubular body. In some embodiments, the expandable tubular body includes iron or a bioerodible iron alloy and at least one surface of the expandable tubular body includes a substantially uniform coating of iron(III) oxide.

Description

Surface Treated Bioerodible Metal Endoprostheses

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provision Application Serial No. 61/316,717, filed on March 23, 2010.

TECHNICAL FIELD

This document relates to surface treated bioerodible endoprostheses, and more particularly to surface treated bioerodible stents.

BACKGROUND

The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprostheses include stents, covered stents, and stent-grafts.

Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen. The expansion mechanism can include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable

endoprosthesis. The balloon can be inflated to deform and to fix the expanded

endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn. In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.

It is sometimes desirable for an implanted endoprosthesis to erode over time within the passageway. For example, a fully erodible endoprosthesis does not remain as a permanent object in the body, which may help the passageway recover to its natural condition. Bioerodible endoprostheses can be formed from polymeric material, such as polylactic acid, or from metallic material, such as magnesium, iron, or alloys thereof.

SUMMARY

An endoprosthesis is described that includes an expandable tubular member defined by a plurality of struts. The expandable tubular body includes a bioerodible metal having at least a first portion and a second portion. The first portion has a thermally altered microstructure. The second portion has a wrought microstructure. The first portion forms at least a portion of an outer surface of the expandable tubular body. In some embodiments, the thermally altered microstructure is a cast microstructure. The thermally altered microstructure can include dendritic grains.

In another aspect, an endoprosthesis is described that includes an expandable tubular member defined by a plurality of struts. At least one strut includes a bioerodible metal that has at least a first surface region and a second surface region. The first and second surface regions have different surface oxide compositions. The bioerodible metal can be iron or an alloy thereof. In some embodiments, the first surface region has a higher percentage of ferrous oxide (iron(II) oxide) than the second surface region. The first surface region can include a thermally altered microstructure (e.g., a cast

microstructure) and the second region can include a wrought microstructure. In some embodiments, the second surface region includes ferroso ferric oxide (iron (II, III) oxide). The first surface region can be essentially free of ferroso ferric oxide. In some embodiments, the second surface region can include ferric oxide (iron(III) oxide). The first surface region can be essentially free of ferric oxide.

The second surface region can include a sulfide of the bioerodible metal. In some embodiments, the first region is essentially free of sulfides. For example, the bioerodible metal can be iron or an alloy thereof and the sulfide can be iron (II) sulfide. In still another aspect, an endoprosthesis is described that includes an expandable tubular body defined by a plurality of struts. The expandable tubular body includes iron or a bioerodible iron alloy with at least one surface of the expandable tubular body includes a substantially uniform coating of iron(III) oxide.

The endoprosthesis can be, for example, a stent.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary stent.

FIG. 2 illustrate exemplary environment for implanting constituents into select regions of a stent.

FIG. 3 depicts a close-up of a laser hole drilled stent.

FIG. 4A depicts the results of depositing a control stent, a PHI treated stent, a laser blasted stent, and a laser hole drilled stent in SBF solution for 28 days.

FIGS. 4B and 4C depict the corrosion rates for control stents, laser blasted stents, and laser hole drilled stents.

FIG. 5 depicts the results of a non-treated bare metal Fe-2.3Mn stent (A), a laser treated Fe-2.3Mn stent having a coating of hematite (B), a thermally oxidized Fe-2.3Mn stent having a coating of hematite (C), and a saline induced predegraded Fe-2.3Mn stent having a coating of hematite (D).

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A stent 20, shown in Fig. 1, is discussed below as an example of one

endoprosthesis according to the instant disclosure. Stent 20 includes a pattern of interconnected struts forming a structure that contacts a body lumen wall to maintain the patency of the body lumen. For example, stent 20 can have the form of a tubular member defined by a plurality of bands 22 and a plurality of connectors 24 that extend between and connect adjacent bands. During use, bands 22 can be expanded from an initial, small diameter to a larger diameter to contact stent 20 against a wall of a vessel, thereby maintaining the patency of the vessel. Connectors 24 can provide stent 20 with flexibility and conformability that allow the stent to adapt to the contours of the vessel. Other examples of endoprostheses can include covered stents and stent-grafts.

Stent 20 includes one or more struts including a bioerodible metal adapted to erode under physiological conditions. The bioerodible metal portion can include a bioerodible metal selected from magnesium, zinc, iron, and alloys thereof. In some embodiments, the bioerodible metal portion includes iron (e.g., substantially pure iron or iron alloy). Iron alloys can include at least 65% iron. For example, the bioerodible metal portion can include a bioerodible iron alloy that includes up to twenty percent

manganese, up to 10 percent silver, and up to five percent carbon. In some embodiments, the bioerodible metal portion includes an alloy including at least 97 weight percent iron and 2.3 weight percent manganese. In other embodiments, the bioerodible metal portion can be 99.9 weight percent pure iron.

One or more of the struts can include surface regions having different surface oxide compositions. For example, a strut can include a first surface region and a second surface region. Different surface oxide compositions in different surface regions can facilitate erosion of the strut when the stent is implanted in a physiological environment by providing localized surface regions that act as anodes and/or cathodes in localized corrosion processes. A surface that is non-homogeneous in surface oxides can provide corrosion initiation sites that accelerate the bioerosion process when the stent is implanted within a physiological environment.

The surface oxide composition of a bioerodible metal can be altered by implanting chemical constituents that displace oxides into the surface of the bioerodible metal. For example, sulfur can be implanted into one of the surface regions of the strut to create sulfides of the bioerodible metal. The implanted sulfur or other constituents can displace the surface oxides normally present at the surface of the bioerodible metal and thus result in different regions having different surface oxide compositions when only some regions are treated. For example, iron and iron alloys typically include a thin surface of ferrous oxide (iron (II) oxide) that can be displaced by implanting sulfur ions.

By implanting sulfur ions into a second surface region of an iron stent strut, a first surface region can be formed having a higher percentage of ferrous oxide than a second region including iron (II) sulfide. Different constituents can be implanted into different surface regions using a number of techniques, including plasma ion immersion implantation (PHI) or beamline ion implantation of boron ions into the surface of the stent. Masking can also be used to prevent the implantation of constituents into all surfaces of the stent strut.

The surface oxide composition of a bioerodible metal can also be altered by depositing clusters of different oxide species that impact the corrosion of the bioerodible metal into the surface of the bioerodible metal. For example, ferroso ferric oxide (iron (II, III) oxide) can be deposited by laser onto a second surface region. The ferroso ferric oxide can have a perovskite and/or spinal chemical structure. For example, by masking a first surface region, the first surface region can be essentially free of ferroso ferric oxide and/or ferric oxide, while the second surface region includes ferroso ferric oxide.

Fig. 2 illustrates an exemplary environment for using PHI to implant sulfur into selected surface regions of selected struts of stent 20. PHI circumvents the conventional line-of-sight restrictions, thus alloys a fast and efficient surface modification of complex- shaped there-dimensional objects. In order to perform PHI, stent 20 is inserted into a chamber 50. In some embodiments, the stent is positioned within a shield 62 that blocks predetermined areas to result in first surface regions that will not be treated. Although shown as covering the majority of the stent 20, shield 62 can in some embodiments only cover smaller portions of the stent 20. In some embodiments, the shield is configured to cover surfaces other than the abluminal surface of the stent. In still other embodiments, the entire stent is treated, and a shield is not used. Chamber 50 is a vacuum chamber containing plasma 56. The vacuum is created by drawing a vacuum through port 54. Plasma 56 contains ions to be implanted into stent 20. Stent 20 is pulsed repeatedly with high negative voltages from pulser 58. As a result of the pulses of negative voltages, electrons are repelled away from stent 20 and positive ions 60 are attracted to the negatively charged stent 20. As a result, positive ions will strike all the exposed surfaces of stent 20 and be embedded in and/or deposited onto second surface regions of stent 20. PHI can be performed at temperatures ranging between 110°C and 400°C In some embodiments, shield 62 could be metal, wherein an electric contact would be formed between the shield 62 and stent 20 in order to provide the negative voltage pulses. In this embodiment, pulser 58 would be electrically coupled to the shield 62. Other suitable materials such as polymers could also be used as the shield 62. Stent 20 is positioned within a shield 62, which blocks predetermined areas to result in first surface regions that will not include sulfides of the bioerodible metal.

In some embodiments, reactive positive sulfur ions are used. Reactive platinum or palladium ions can also be embedded using PHI. Depositing reactive elements can result in a stent that corrodes by galvanic corrosion, can also impart compressive stresses in the metal, and/or can also alter the oxide composition in one or more regions of the stent, as discussed above. Moreover, implanting elements can result in a heating of the outer surface to a temperature sufficient to melt and recast the outer surface of the bioerodible metal. For example, for reactive applications a dosage of between lei 6 and lei 7 ions/cm2 at implantation voltages of 10 kV is sufficient to create the regions having a recast structure having a lower oxide concentration.

PHI treatment with inert gasses (e.g., Argon, Helium, and Krypton) can also modify the surface chemistry and microstructure of a surface of a bioerodible metal. For example, inert gasses can add or remove elemental species to/from the alloy, create porosity, or create new phases in the microstructure. Inert atoms (e.g., Ar, He, or Kr) can be embedded by PHI in the metal matrix to cause a very high compressive stress. The compressive stress can result in a higher corrosion rate. For example, inert elements can be implanted at dosages of up to about lei 8 ions/cm2 at a pulse violated of between 20 and 40 keV to create stresses in the iron material. In some embodiments, variations in the plasma conditions can be used to provide varying stresses in different portions of the stent.

In some embodiments, the stent includes a first portion having a thermally altered microstructure and a second portion having the original wrought microstructure. The thermally altered microstructure is produced by heating the metal to a temperature that produces a single phase solid solution and/or is within the solidus-liquidus range or above the melting temperature, and then removing the heat source so that the metal cools to ambient temperature. The thermally altered microstructure includes reduced texture anisotropy, reduced dislocation density from cold working, amorphous metal, refined grains, dendritic grains, or single phase conditions. In some embodiments, the thermally altered microstructure also contains a layer or layers of parent material that reacted with the environment present during the heating; e.g., carburized, decarburized, oxidized, or nitrided metal. The thermally altered microstructure can be formed at least a portion of an outer surface of the stent. In some embodiments, the outer surface of the stent is entirely formed of a thermally altered microstructure (i.e., the thermally altered microstructure encapsulates the wrought microstructure portion of the stent). In some embodiments, the abluminal surface of the stent is formed of a thermally altered microstructure, while other surface portions have the wrought microstructure. In some embodiments, the stent includes a continuous body formed of a wrought microstructure with a plurality of isolated regions having a thermally altered microstructure defining portions of the outer surface of the stent. In some embodiments, the thermally altered microstructure is a cast microstructure. In some embodiments, the thermally altered microstructure includes dendritic grains.

The thermally altered microstructure can be formed by the methods discussed above, e.g., by PHI implanting of reactive or inert species. For example, for reactive applications a dosage of between lel6 and lei 7 ions/cm² of platinum or palladium at implantation voltages of 10 kV is sufficient to create the regions having a recast structure having a lower oxide concentration. Additionally, a thermally altered microstructure can also be formed by applying laser energy onto an annealed wrought stent surface. Weld spots or bands on selected surface regions of the stent strut can also be used to create different surface regions having different surface oxides concentration and/or

compositions. Laser blasting can also be used to form an even layer of thermally altered material. In some embodiments, a laser beam is used to slightly melt the abluminal metal surface of a stent in a controlled atmosphere. In other embodiments, all surfaces of a stent can be laser blasted. The atmosphere of a laser blasting operation can be inert (e.g., argon, nitrogen) or reactive (e.g., oxygen). The cooling rate and the atmosphere may impact the resulting microstructure (thermally altered or original wrought) and the oxide compositions of the surface. The thermally altered layer can have a thickness of between 0.05 micrometers and 10 micrometers. This thermally altered layer can be porous and have a wavy topography due to the presence of the thermally altered grain morphology (e.g., a morphology including dendritic grains).

In some embodiments, a laser can be used to form nano or micro sized superficial weld spots or bands on the surface of an iron or iron alloy stent to form surface regions having thermally altered microstructures (e.g., cast microstructures) having a lower percentage of ferrous oxide (iron (II) oxide) than other surface regions. For example, superficial weld spots would be of a width that is from 0.1 to 0.8 of the strut width and a depth of 1 micron to the full wall thickness; i.e., a superficial weld or fully penetrating weld.

In other embodiments, a laser can be used to drill holes into the surface. For example, laser blasting in a controlled atmosphere with short pulses (e.g., with pico second lasers) can also be used to form drill holes in the surface. FIG. 3 depicts a stent having laser drilled holes formed using a pico second laser. In some embodiments, the laser formed holes have a diameter of between 10 nm to 20 micrometers in diameter and 1 to 20 micrometers in depth. For example, the laser formed holes can have a diameter of about 1 micrometer. Laser formed holes can also facilitate crevice corrosion. The inside surface of the laser formed holes can include the thermally altered microstructure, which includes dendritic grains. The untreated regions of the iron or iron alloy stent can have a wrought microstructure. In other embodiments, laser formed holes can be formed in a stent that is pretreated to have a thermally altered microstructure over its entire surface. In other embodiments, the abluminal surface of the stent can be treated to have a thermally altered microstructure and have laser formed holes in the abluminal surface and/or other surfaces, while other surfaces retain a wrought microstructure.

Laser formed holes can all be of the same size or can be multiple sizes. For example one laser pass could be programmed to produce an array 10 micrometer diameter x 5 micrometer deep holes where each hole is spaced apart from adjacent holes (center-to-center) by 20 micrometers. This array can be centered on the strut peak surfaces where surface area is at a maximum, but not extend all the way to the strut edges. In another embodiment, a laser pass may be programmed to produce an array of 50 nm diameter x 1 micrometer deep holes surrounding the first array of larger holes. In some embodiments, the smaller holes are positioned along strut edges and larger holes are positioned therebetween, which can minimize stress concentrators along the strut edges where stent deformation strains are concentrated. This arrangement may facilitate corrosion more aggressively in the central regions of strut peak faces and less aggressively along strut edges to avoid early fracture of deteriorating struts.

The laser formed hole shapes could be round, square, rectangular, or oblong. Shapes other than round can be produced by shielding the stent surface from the laser with fixture that contained holes in the desired pattern of shapes and sizes (acting as a laser template). In some embodiments, the shield is made of a material that would not be cut or deteriorated by the impinging laser; e.g., tantalum, niobium, alumina or silica ceramic, or diamond-like carbon coated steel.

The speed and energy of the laser impinging on the stent surface can be set to produce particular depths of microstructural transformation from wrought to thermally altered condition. For example, the laser can be operated to product a depth of microstructural transformation along the sides and bottom of the hole that is 50 nm, 100 nm, 500 nm, 1 micrometer, or 5 micrometers. In some embodiments, the holes can include a cast microstructure having dendritic grains.

In some embodiments, a second laser operating within the same chamber as a primary laser performing the stent hole drilling operation is used to imping upon a noble metal surface (e.g., platinum, palladium, tantalum, or niobium) and producing vaporization of the noble metal. The noble metal vapor can become somewhat entrained in the primary hole drilling environment such that a small amount of noble metal would be mixed in with the modified metal of the stent. This enrichment can contribute to the corrosion rate enhancement by galvanic corrosion. In some embodiments, this enrichment can result in the presence of 0.5 to 5 atomic percent of the noble metal in a discontinuous network on the stent surface and in the laser formed hole surfaces up to or less than the depths of the microstructural modification.

Having an outer surface having a thermally altered microstructure (e.g., a cast microstructure) and other portions having a wrought microstructure can result in a galvanic potential difference that can accelerate the corrosion of the stent. Referring to FIGS. 4A-4C, the presence of a thermally altered microstructure on an outer surface of a stent made of a Fe-2.3Mn alloy increases corrosion rate relative to an untreated Fe-2.3Mn alloy stent. For example, FIG. 4A depicts the results of immersing a control untreated Fe- 2.3Mn stent (i), a PHI treated iron alloy stent (ii), a laser blasted Fe-2.3Mn alloy stent (iii), and a Fe-2.3Mn alloy stent having laser drilled holes (iv). The top picture shows the appearance of each stent prior to its immersion in the SBF solution at 37°C and the bottom picture depicts the appearance of each stent after 28 days in the SBF solution at 37°C. The SBF solution has the composition shown in Table 1.

Table 1: Simulated Body Fluid (SBF) Ingredients

As shown, the untreated Fe-2.3Mn control stent did not have noticeable corrosion, while the treated stents all had a noticeable mass reduction. FIG. 4B depicts the mass loss plotted over time for stents (i), (iii), and (vi). FIG. 4C depicts the corrosion rates as a thickness lost per year for two control stents, 2 laser hole drilled stents, and a laser blasted stent.

One or more of the struts of stent 20 can include, in other embodiments, a coating of an iron(III) oxide. For example, the iron(III) oxide can be a ferric oxide (e.g., hematite). In some embodiments, the iron(III) oxide can be an iron(III) oxide-hydroxide (e.g., goethite, akaganeite, lepidocrocite, feroxyhyte, ferrihydrite, schwertmannite, or high-pressure FeOOH). The ferric oxide coating can be substantially uniform. The iron(III) oxide coating can surround a stent strut. In some embodiments, the iron(III) oxide coating can encapsulate the iron or bioerodible alloy thereof. In some

embodiments, the iron(III) oxide coating is substantially free of phosphorus (i.e., less than 5 weight percent phosphorus). In some embodiments, the coating can consist essentially of one or more iron(III) oxide (i.e., have at least 80 weight percent of iron(III) oxide). In some embodiments, the iron(III) oxide coating encapsulates a continuous body of the iron or the bioerodible iron alloy.

When exposed to a physiological environment, iron may react (e.g., oxidize or mineralize) and deteriorate into a variety of compounds. These compounds may slow the corrosion process by acting as a barrier between the iron and the environment. This process is sometimes known as pacification. A coating of iron(III) oxides, however, can reduce the passivation of the iron and thus result in a faster and more uniformly corroding iron or iron alloy stent. Accordingly, a purposeful surface treatment to produce a ferric oxide coating, which will reduce the spontaneous creation of other oxides or

mineralization of the outer surface of the iron or bioerodible iron alloy stent.

A coating thickness of at least 10 nm can avoid mineralization passivation in an in-vivo environment. In some embodiments, the iron(III) oxide coating has a thickness of between 10 nm and 10 μιη. For example, coating of ferric oxide can be formed by a variety of processes, including laser treating, predegradation in a controlled solution (e.g., an NaCl solution), and thermal oxidation. Predegradation in a NaCl solution allows the iron to corrode (i.e., oxidize) without the presence of passivating ions. Thermal oxidation can be performed by placing an electropolished stent into an oven heated to between 205°C and 270°C for between 1 minute and 15 minutes. The presence of ferric oxide (e.g., hematite) can be determined by Raman Spectroscopy or X-Ray diffraction and the thickness can be measured by cross-sectioning.

Bare metal stents composed of a Fe-2.3Mn alloy, without any special surface treatment, were implanted in pig coronary arteries for 30 days. These stents exhibited very little evidence of corrosion and no notable rust visible around the stents. Using SEM energy dispersive spectroscopy (EDS) and Raman spectroscopy, an iron-oxygen- phosphorus mineral compound (e.g., Vivianite) and ferric oxide (e.g., hematite) were identified as the degradation products. The phosphorus containing compound may be the result of phosphorous present in the blood stream.

As shown in FIG. 5, in-vitro degradation tests of stents including the Fe-2.3Mn alloy and having a layer of hematite demonstrated a higher corrosion rate than an untreated Fe-2.3Mn alloy stent when placed in a SBF solution. Each stent was submerged in the SBF solution having the composition noted above for 28 days, at 37°C. The hematite coating was created on the stents by three different processes: laser treating (B), via thermal oxidation (C), and predegrading in NaCl solution (D). [The laser treatment was performed using Pico lasers (TRUMPF Trumicro 5050) set at: P = 0.5W - 3W, pulse frequency 50 kHz, pulse energy 5-35 μΐ, spot distances = 25 - 40 μιη, wave length 1030 nm, pulse length 12 ps; optics: F-Theta f=53mm, focus diameter 12 μιη. The array was axially scanned through galvanometer scanner with 14 mm inlet aperture, and a rotation was applied after scanning axially.

Thermal oxidation was performed in a Thermo-gravimetric analyzer (TGA) for 4 minutes at 232°C. Predegrading in the 0.9% NaCl solution creates hematite by allowing the iron to corrode without being exposed to passivating ions. As shown in FIG. 6, the non-treated stent (A) remained more intact and has less corrosion than each of the hematite coated stents (B, C, and D).

The presence of the non-passivated coating including an iron(III) oxide may permit an implanted stent may permit endothelialization to occur prior to the formation of significant amounts of phosphorous containing compounds along the outer surfaces of the stent. Once endothelialized, the iron(III) oxide coated stent can continue to corrode in an environment having a lower concentration of phosphorous than the blood stream.

Accordingly, the iron(III) oxide coating can result in a stent that erodes at a faster rate even after the iron(III) oxide coating has eroded away.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other embodiments are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. An endoprosthesis comprising an expandable tubular body defined by a plurality of struts, the expandable tubular body comprising a bioerodible metal having at least a first surface region and a second surface region, the first and second surface regions having different surface oxide compositions.

2. The endoprosthesis of claim 1, wherein the bioerodible metal is iron or an alloy thereof.

3. The endoprosthesis of claim 2, the first surface region has a higher percentage of ferrous oxide (iron(II) oxide) than the second surface region.

4. The endoprosthesis of claim 3, wherein the first surface region comprises a cast microstructure and the second region comprises a wrought microstructure.

5. The endoprosthesis of claim 2, wherein the second surface region comprises ferroso ferric oxide (iron (II, III) oxide).

6. The endoprosthesis of claim 5, wherein the first surface region is essentially free of ferroso ferric oxide.

7. The endoprosthesis of claim 2, wherein the second surface region comprises ferric oxide (iron(III) oxide).

8. The endoprosthesis of claim 7, wherein the first surface region is essentially free of ferric oxide.

9. The endoprosthesis of claim 7, wherein the first surface region comprises hematite.

10. The endoprosthesis of claim 1 , wherein the second surface region comprises a sulfide of the bioerodible metal.

11. The endoprosthesis of claim 10, wherein the first region is essentially free of sulfides.

12. The endoprosthesis of claim 10, wherein the bioerodible metal is iron or an alloy thereof and the sulfide is iron (II) sulfide.

13. The endoprosthesis of claim 12, wherein the endoprosthesis is a stent.

14. An endoprosthesis comprising an expandable tubular body defined by a plurality of struts, the expandable tubular body comprising a bioerodible metal having at least a first portion and a second portion, the first portion having a thermally altered microstructure, the second portion having a wrought microstructure, the first portion forming at least a portion of an outer surface of the expandable tubular body.

15. The endoprosthesis of claim 14, wherein the bioerodible metal is iron or an alloy thereof.

16. The endoprosthesis of claim 14, the first portion encapsulates the second portion.

17. The endoprosthesis of claim 14, wherein the first portion defines the abluminal

surface of the expandable tubular body.

18. The endoprosthesis of claim 14, wherein the tubular body comprises a continuous body of the second portion with a plurality of first portions each having a cast microstructure comprising dendritic grains, wherein each first portion defines at least a portion of the outer surface of the expandable tubular member.

19. The endoprosthesis of claim 18, wherein the plurality of first portions comprise holes or cavities each surrounded by the second portion.

20. The endoprosthesis of claim 14, wherein the first portion and the second portion each include surface regions having different surface oxide compositions.

21. The endoprosthesis of claim 14, further comprising a coating overlying the outer surface of the expandable tubular member.

22. The endoprosthesis of claim 14, wherein the endoprosthesis is a stent.

23. An endoprosthesis comprising an expandable tubular body defined by a plurality of struts, the expandable tubular body comprising iron or a bioerodible iron alloy, at least one surface of the expandable tubular body comprising a substantially uniform coating of iron(III) oxide.

24. The endoprosthesis of claim 23, the coating of ferric oxide encapsulati

expandable tubular body.

25. The endoprosthesis of claim 23, wherein the coating comprises ferric oxide.

26. The endoprosthesis of claim 23, wherein the tubular body comprises a continuous body of the iron or the bioerodible iron alloy.

27. The endoprosthesis of claim 23, wherein the expandable tubular body comprises an alloy consisting essentially of iron and about 2.3 weight percent manganese.

28. The endoprosthesis of claim 23, wherein the endoprosthesis is a stent.